Propellant extrusion die

ABSTRACT

An extrusion die for use in producing perforated stick-type propellant includes a die blank having a central passage therethrough, the passage having a tapered entry, an open lattice webbing structure having struts in and spanning the central die passage, and an array of die pins for imparting perforations in material forced through the central passage, each pin having a fixed end attached to the lattice structure and a free end extending parallel to the passage beyond the lattice structure.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates generally to the field of propellantcharges, particularly propellant charges suited to be used in large ormedium caliber projectile ammunition which are made up of extrudedperforated stick-type propellant grains. More particularly, the presentinvention relates to a propellant extrusion die design that incorporatesan array of perforation-forming pins but that eliminates the need for apin plate and enables straight through propellant extrusion therebysubstantially eliminating flexing of the die pins. This further enhancesuniformity in perforation patterns associated with extruded perforatedstick-type propellant shapes extruded through the die and the enhanceduniformity reduces the amount of unburned propellant slivers resultingat burnout. In addition, in accordance with the present invention, diescan be manufactured with pins of any desired cross sectionalconfiguration and thus they can produce any desired perforation shapesin the propellant stick grains. Certain shapes have been found tosignificantly reduce propellant slivers associated with multi-perfpropellant burns.

II. Related Art

The success of all ammunition rounds depends greatly upon theperformance and reproducibility of the performance of the associatedpropellant system. In this regard, those skilled in the art have longsought to control the mass rate of gas generation with predictableprogressive burns. Control of the burn has been enhanced for certaintypes of munitions by the use of perforated extruded stick propellantshapes packed into the munition cartridge to be fired.

Almost all extruded gun propellants have perforations parallel to thelengthwise dimension of the extruded stick grains to provide ballisticprogressivity as the propellant burns. Depending on size andapplication, stick propellants are normally processed with 1, 7, 19 oreven 37 or more perforations (perfs) to enhance progressivity.Controlled progressivity is vital to the performance required by moderngun systems.

Stick propellant is extruded through extrusion tools in the form ofextrusion dies which are designed to produce an extrudate having thedesired shape including the internal voids associated with theperforations. The physical shape, of course, is determined by therequirements of the gun ammunition system. Extrusion dies of the classdescribed are provided with die pins that are used to impart theperforations in the finished propellant grains. Typically, 7 perf gunpropellant grains, for example, are provided with one centralperforation and a single row radial hexagonal pattern of 6 perforationssurrounding the central perforation. Other patterns including 19perforations (a pattern of 1, 6, 12 perfs) and even 37 (a pattern of 1,6, 12, 18 perfs) or more perforations are used in certain propellantdesigns.

Particular limitations regarding the production of stick-type propellantgrains have resulted from limitations associated with the extruding diesthemselves. FIGS. 1(a) and 1(b) depict top and cross sectional views ofa typical 2-piece prior art extrusion die, generally at 10, whichincludes a die body 12 which may be heat treated tool or stainless steeland a pin plate 14 of the same material which nests in the die body atthe entrance to the die when it is assembled in place. An orienting pinfor the pin plate and matching recess in the die body are shown at 15.The pin plate is provided with an array of inlet passages 16 throughwhich propellant must be forced at high pressure (usually >5000 psi) tobe admitted to the die from a supply of propellant to be extrudedupstream of the die. An array of 7 pins is shown at 18 forming a regularhexagon surrounding a central pin. The pins themselves designated as 20are fixed to the pin plate as by being press fit into the plate inopenings at 22, the remainder of the pin 20 being free and extending thelength of the die body 12. The die body 12 is rather wide at the top orentrance to accommodate the pin plate and must be provided with atransitional zone as at 24 which tapers down to the size of the actualextrusion or agate area 26 which defines the cross sectional size of thestick.

Propellant entering the transitional zone 24 through the openings orpassages 16, as can be seen from the drawings, is forced at highpressure to approach and converge on the pin array 18 and thus the diepins 20 from the outside at an angle that approaches perpendicular tothe die pins 20. In the transitional zone 24, the propellant flowsnearly perpendicular to the die pins 20 and this causes flexing of thedie pins. Prior die designs which processed a specific propellantformulation and web size could make some allowances for the predictedpin flexure. However, process variation such as propellant solventcontent (rheology), temperature, extrusion rate, etc. causeunpredictable variations in forces impinging the pins and, thus, changesin the pin flexure.

Variation in propellant die pin flexure has been ultimately manifestedin variation of key physical dimensions such as web size and webdifference (difference between inner and outer web thicknesses which aredesigned to be equal) in the perforated stick grains. Whilemodifications have been made to the dies in an effort to reduce pinstress such as rounding the transition zone and utilizing fewer, largeropenings in the pin plate, they have only met with partial successes andthere remains a long-felt need to improve perforated stick grainpropellant extrusion dies.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a newextrusion die design that solves many problems enumerated above byallowing for secure placement of the die pins without a pin plate. Theresult is a straight-through die design that eliminates thedisadvantages of using a pin plate and reduces inconsistencies inextruded propellant caused by pin flexure.

The present invention provides an extrusion die for use in producingperforated stick-type propellant which includes a die body having acentral passage extending through the body, the passage having an opentapered entry cavity and an open lattice webbing structure within thebody of the die spanning the central die passage. The strut members ofthe open lattice webbing structure extend parallel to the central diepassage and divide but provide very little obstruction to the passage ofmaterial being extruded. An array of die pins for imparting perforationsin material forced through the central passage includes pins each havinga fixed end fixed to the lattice structure and a free end extendingparallel to the passage beyond the lattice structure such that thematerial being extruded flows parallel to and around the pins.

The die of the invention may be formed as unitary structures from a dieblank utilizing both conventional machining and electron dischargemachining (EDM) techniques. Thus, after the outside of the blank ismachined and holes are drilled in the blank corresponding to openings inthe lattice structure, EDM may be used to cut out the lattice web,together with the desired array of pins with great accuracy. The pinsmay be formed at the time the open lattice web is machined, or they maybe separately fabricated and attached as by press fitting into recessesprovided in the open lattice webbing itself. If separate pins are to beinserted, a slightly thicker webbing is used.

The preferred material of construction for the extrusion die of theinvention is precipitation hardened stainless steel, possibly 15-5 PH or17-4 PH stainless steel. Separately manufactured pins may be constructedof hardened tool steel. The die passage surrounding the vicinity of thepins preferably may be slightly tapered in accordance with reforming thepropellant stick after it becomes segmented when it encompasses the openlattice web structure.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, wherein like reference characters depict like partsthroughout the same:

FIG. 1(a) is a top view of a prior art propellant extrusion die showingthe top of the die pin and indicating the central pin pattern;

FIG. 1(b) is an elevational view partially in section of the extrusiondie of FIG. 1(a);

FIG. 2 is a top view of an extrusion die fabricated in accordance withthe present invention;

FIG. 3 is an elevational view partially in section of the extrusion dieof FIG. 2, taken along lines 3-3 of FIG. 2;

FIG. 4 is an elevational sectional view taken substantially along lines4-4 of FIG. 2;

FIG. 5 is a greatly enlarged view showing one pin detail of an extrusiondie fabricated in accordance with the invention;

FIG. 6 is a greatly enlarged top perspective view of the die constructedin accordance with the invention; and

FIG. 7 is a greatly enlarged bottom perspective view of the die of FIG.6 showing the area of pins as integral with lattice webbing struts inthe area of free length of the pins.

DETAILED DESCRIPTION

The extrusion die of the present invention will now be described withrespect to a specific embodiment, however, the descriptions containedherein are intended to present examples of embodiments of the inventionand examples of methods of making the embodiments of the invention andare not meant to be limiting with regard to the scope of the inventionin any manner. In this regard, an important aspect of the inventioninvolves the provision of a straight-through extrusion arrangement whicheliminates the need for propellant to encounter the perforation pinsfrom the side. By enabling extrusion directly into the web, allpropellant motion is substantially parallel to the pins which isbeneficial both to product quality and pin life.

FIGS. 2-4 depict one embodiment of the extrusion die of the inventionwhich is depicted generally by the reference character 40. The die maybe of a single piece unitary construction and includes an upstream ordie body entry opening 44 which is generally tapered at an acute anglenarrowing down to the entry of the main body or agate section of the die46 which contains the open lattice webbing which includes a center 48and a series of relatively thin radial struts connecting the center withthe inner wall of the die as at 50 which form the open lattice webbingstructure through which the extruded propellant passes during theextrusion process. Each of the webbing struts 50 includes an enlarged,raised shaped area as at 52 (see also FIGS. 6 and 7) that is in theshape of and at the radial location of a perforation pin 54. The pins 54preferentially end a short distance before the end of the die to preventpin damage. A minor recess may be machined into the bottom of the die asat 56, if desired.

FIG. 5 shows a greatly enlarged alternate pin detail in which a circularhexagonal array of substantially trapezoidal shaped pins 70 are machinedas integral extensions of the lattice webbing struts 50 and the centerpin 72 is a round press fit, separately manufactured pin. Note that theinner and outer bases of the trapezoidal pin are indicated by and areco-incident with concentric circles 74 and 76, at least the outer,longer base of each trapezoid being of an arcuate shape to coincide withthe round outer dimension of propellant extruded through the subjectdie.

The trapezoid is one non-round shape that has been used to greatlyenhance burn progressivity (by elimination of slivers) in certainpropellant sticks. The use of non-round shaped extrusion pins andpropellant stick grains made with them is described in greater detail inco-pending application Ser. No. 10/______, filed of even date andassigned to the same assignee as the present invention, entitled“PROPELLANT EXTRUSION USING SHAPED PERFORATION PINS”. That applicationis hereby incorporated by reference herein in its entirety for anypurpose.

FIGS. 6 and 7 show greatly enlarged top and bottom perspective views ofan embodiment of the invention which utilizes 7 round pins to create a7-perf extruded stick. FIG. 7, particularly, depicts the free ends ofthe pins extending beyond the open lattice webbing struts 50. As can beseen from the figures, and particularly, FIG. 4, the area of the openlattice webbing between its beginning at 58 and ending at 60 is tapered.The taper is normally between about 9° and 11°, but may be varied asdesired. The taper slightly constricts the propellant that has beensegmented in moving past the struts 50 of the open lattice webbing ofthe die so that it more readily re-forms a single stick in the lower orexit die area 62. The area 62, of course, is in the area of the freelength of the pins 54. It should be noted that very little movement ofthe propellant is required in the direction perpendicular to the pinsduring the extrusion process.

The dies 40 can be made from a single piece of stainless steel firstmachined using conventional machining techniques where possible toachieve the desired outer surface and inner entry taper, and if desired,an outlet recess. In the case of a 7-perf system, 6 holes are thendrilled in the blank corresponding to the void areas between the webbingstruts and an EDM device using a wire cutout system is utilized to makethe web and, if desired, the pins so that the entire device is integralwith the original blank. This technique allows highly accurate web andpin structures to be produced. As indicated before, the material of thedie blank is preferably heat treated stainless steel as it must be amaterial which is corrosive resistant when exposed to various propellantcompositions, caustic cleanout material and other materials associatedwith processing the propellant. Such techniques are known and can createwebs or pins of any desired shape or thickness.

While this technique has been found quite successful, the applicantscontemplate that other hybrid techniques might also be employed. Forexample, the center pin is typically manufactured separately and pressfit into a central tapered opening as shown at 64 in FIG. 4. Inaddition, the radially distributed pins of any desired shape may befabricated separately and added to the web after other machining iscompleted. This requires slightly enlarged shaped areas for receivingpins as at 52 to be created on the struts of the web and press fit orother techniques employed to implant the pins such as that described forthe center pin.

The straight-through extrusion die enables great improvements to beachieved in geometric stability of the pin pattern. Using the techniquesof the present invention, pin flexure associated with the extrusionprocess has been reduced by 75-80% from conventional dies. Variations inthe web thickness of propellant extruded through the dies have beengreatly improved (reduced), i.e., from about 7.0%, which has beencommonly encountered with prior dies, to 4% or less utilizing the diesof the present invention. The dies of the present invention may achievea web uniformity variance as little as 2-3% in some cases. In addition,in recent extrusion tests, dies in accordance with the present inventionhave experienced a first pass yield of 90% or greater of usable materialversus no more than 80% with prior conventional dies.

This invention has been described herein in considerable detail in orderto comply with the patent statutes and to provide those skilled in theart with the information needed to apply the novel principles and toconstruct new such specialized components as are required. However, itis to be understood that the invention can be carried out byspecifically different devices and that the various modifications, bothas to the equivalent details and operating procedures can beaccomplished without departing from the scope of the invention itself.

1. An extrusion die for use in producing perforated stick-typepropellant comprising: (a) a die blank having a central passagetherethrough, said passage having an unrestricted tapered entry; (b) anopen lattice webbing structure beyond said tapered entry in said centralpassage for passing extruding propellant, said webbing structureproviding struts in and spanning said central die passage; and (c) anarray of die pins carried by said webbing structure arranged in apattern for imparting a pattern of perforations in material forcedthrough said central passage, each pin having a fixed end attached tosaid lattice webbing structure and a free end extending parallel to saidpassage beyond said webbing structure.
 2. An extrusion die as in claim 1wherein said die is formed as a unitary structure.
 3. An extrusion dieas in claim 1 wherein said central passage is tapered slightly in thevicinity of said lattice webbing structure.
 4. An extrusion die as inclaim 1 wherein said open lattice structure is machined in said centralpassage.
 5. An extrusion die as in claim 2 wherein said open latticestructure is machined in said central passage.
 6. An extrusion die as inclaim 1 wherein at least some of the pins are formed integrally withsaid open lattice webbing structure.
 7. An extrusion die as in claim 4wherein at least some of the pins are formed integrally with said openlattice webbing structure.
 8. An extrusion die as in claim 1 wherein oneor more of said pins is separately manufactured and fixed to saidlattice webbing structure.
 9. An extrusion die as in claim 8 whereinseparately manufactured pins are press fit into openings provided insaid lattice webbing structure.
 10. An extrusion die as in claim 1wherein one or more of said pins is of a non-round cross section.
 11. Anextrusion die as in claim 1 wherein the number of pins arranged in saidpattern is selected from 7, 19 and 37 and wherein said pattern includesa central pin.
 12. An extrusion die as in claim 11 wherein the number ofpins is
 7. 13. An extrusion die as in claim 4 wherein said machiningincludes electron discharge machining.
 14. An extrusion die as in claim5 wherein said machining includes electron discharge machining.
 15. Anextrusion die as in claim 1 wherein the area of the open lattice webbingstructure is tapered slightly to enhance reforming of extruded materialinto sticks.
 16. A method of extruding perforated stick-type propellantincluding the step of: extruding propellant through the die blank ofclaim 1, said propellant passing through said die blank maintaining adirection substantially parallel to said pins along the length thereof.